1. Field of the Invention
The invention relates to charged-particle optical systems such as electron and ion probe-forming systems, electron and ion microscopes, and electron and ion microanalyzers. It relates particularly to high resolution scanning electron microscopes, high resolution scanning transmission electron microscopes, fixed-beam transmission electron microscopes, and to high resolution scanning ion microscopes, scanning ion microanalyzers, and further to focused ion beam systems used for sample erosion. It also relates to charged-particle optical systems used for lithography and mask writing.
2. Description of Prior Art
Round electromagnetic and electrostatic lenses normally used in electron and ion beam optical systems, hereafter referred to as charged-particle optical systems, suffer from unavoidable aberrations including spherical aberration and chromatic aberration. The aberrations can be overcome by aberration correctors comprising non-round lenses such as quadrupole and octupole lenses, as for instance described in an article entitled xe2x80x9cTowards sub-xc3x85 electron beamsxe2x80x9d by O. L. Krivanek, N. Dellby and A. R. Lupini published in the journal Ultramicroscopy (volume 78, page 1-11), hereby incorporated by way of reference, and in an article entitled xe2x80x9cProgress in aberration-corrected scanning transmission electron microscopyxe2x80x9d by N. Dellby et al. published in the Journal of Electron Microscopy (volume 50, page 177-185), hereby also incorporated by way of reference. They can also be corrected by aberration correctors comprising sextupole lenses, as for instance described by Crewe in U.S. Pat. No. 4,414,474 and Rose in U.S. Pat. No. 5,084,622, hereby incorporated by way of reference.
Aberration correctors comprising quadrupole and octupole lenses have several important advantages compared to correctors comprising sextupole lenses. The primary effect of an octupole lens is to act directly on spherical aberration, which is a dominant aberration that unavoidably affects all round-lens charged-particle systems. By contrast, sextupole lenses, which are responsible for correcting spherical aberration in sextupole correctors, have the primary effect of producing strong three-fold astigmatism, a second order aberration. Correction of spherical aberration by a sextupole lens arises only as a by-product of the primary action, in a sextupole of extended length. As a result, at least two sextupoles must be used in a sextupole corrector, with the second sextupole appropriately energized so that it nulls three-fold astigmatism introduced by the first sextupole. Small misalignment between the two sextupoles, for instance introduced by a small variation of the strength of an alignment dipole, or a mechanical movement of the supporting column, results in the cancellation of the strong three-fold astigmatism not being complete. It typically leads to a large parasitic first order astigmatism, which adversely affects the attainable resolution. As a result, correctors utilizing sextupole lenses need to maintain higher stability and precision than quadrupole-octupole correctors in order to reach equivalent resolution.
Another important advantage of quadrupole-octupole corrector systems is that their quadrupoles may be constructed in such a way that they have negative chromatic aberration. As a result, a quadrupole-octupole corrector may be made to correct chromatic aberration in addition to geometric aberrations such as third order spherical aberration. Chromatic and spherical aberrations are typically the two most important aberrations of an uncorrected charged-particle optical system, and simultaneous correction of both the aberrations leads to considerably improved performance compared to optical systems that only correct one or the other aberration. By comparison, a sextupole corrector cannot correct chromatic aberrations unless it also incorporates extra quadrupole lenses, which do not lend themselves naturally to incorporation in a sextupole corrector.
Yet another major advantage of a quadrupole-octupole corrector system is that it does not need to use any round lenses within the corrector part of the system, since this part produces the required first order trajectories purely with quadrupole lenses. In contrast, sextupole correctors usually employ round lenses within the corrector. Magnetic quadrupole lenses typically consume less than 1% of the power required by round lenses of comparable focal length, and are therefore very efficient. As a result, large thermal loads which require water cooling, and which can easily lead to long-term drifts, are avoided by the use of quadrupole lenses rather than round lenses. This simplification leads to increased long-term stability, and simplified operation and maintenance of the charged-particle optical apparatus. A further advantage of quadrupole lenses is that unlike round lenses, they produce no image rotation, and this can lead to further simplifications in systems which use quadrupole lenses rather than round lenses disposed between strong multipole lenses such as sextupoles or octupoles.
On the other hand, a well-known advantage of sextupole correctors is that sextupoles correct spherical aberration uniformly in all directions that are transverse to the direction of travel of the charged-particle beam. This property makes possible sextupole correctors in which the action of the sextupoles is projected into the coma-free plane of the principal lens of the charged-particle apparatus, typically known as the objective lens. Such systems have minimized fifth order axial aberrations and also small field aberrations. They are not, however, completely free of fifth order aberrations, since six-fold astigmatism of fifth order typically remains uncorrected. An example of such a corrector system was described in an article entitled xe2x80x9cOn the fifth order aberration in a sextupole corrected probe forming systemxe2x80x9d by Z. Shao, published in the Review of Scientific Instruments (volume 59, page 2429-2437), hereby incorporated by way of reference.
Octupoles affect spherical aberration differently in different transverse directions. As a consequence, a quadrupole-octupole corrector typically comprises three or more octupole lenses, and the effective plane in which undesirable third order deviations of a ray leading through an optical system are corrected, hereafter called the correction plane, typically depends on the transverse direction of the ray. In such a system, it is typically impossible to project all the planes in which aberration correction takes place into a coma-free plane of the objective lens. Not projecting the correction planes into the coma-free plane of the objective lens leads to combination aberrations such as fifth order spherical aberration, two-fold astigmatism of fifth order and four-fold astigmatism of fifth order. These aberrations limit the improvement in the resolution performance of a typical charged-particle optical apparatus originally suffering from spherical aberration of third order, but corrected by a quadrupole-octupole aberration corrector, to two to three times better than the uncorrected performance, as described for instance in N. Dellby et al. in an article in the Journal of Electron Microscopy (reference cited above).
Most users of charged-particle optical systems would find significant advantage in an apparatus and a method allowing for complete cancellation of all fifth order aberrations with a quadrupole-octupole corrector system. Such an apparatus and method would improve the resolution by more than a factor of five compared to an uncorrected optical system, and more than a factor of two compared to an optical system utilizing an aberration corrector suffering from fifth order combination aberrations. The users would find significant advantage in the resultant improved resolution charged particle images that such an apparatus and method would provide, thereby enabling them to explore the structure of matter with hitherto unattainable resolution and sensitivity. They would also find advantage in the smaller diameter and more intense charged particle probes that such an apparatus and method would allow, thereby enabling users to explore matter at a higher resolution than hitherto possible, and to nano-machine matter with hitherto unattainable precision. They would further find a significant advantage in the greater immunity of a quadrupole-octupole corrector to misalignments and other instabilities than is typical with a sextupole-type corrector, the decreased power requirements of a quadrupole-octupole corrector compared to a sextupole corrector incorporating round lenses, and the absence of image rotation in a quadrupole-octupole corrector. Moreover, they would find a great advantage in the fact that a quadrupole-octupole corrector can also be made to correct chromatic aberration, thereby leading to a significant further improvement in the resolution of charged-particle images and to smaller charged-particle probes than can be attained with correctors correcting only geometric aberrations.
Accordingly, several of the objects and advantages of the present invention are: to provide a method and an apparatus for a quadrupole-octupole corrector charged-particle optical system that is able to correct geometric aberrations up to fifth order, and to provide an apparatus and a method for correcting geometric aberrations up to fifth order while at the same time correcting chromatic aberration or aberrations. Another object and advantage of the present invention is to provide these aberration-correcting capabilities in an apparatus relatively insensitive to misalignments, instrumental drifts and other instabilities, and to provide them in an apparatus not using any high-power round lenses within the corrector part of the total apparatus, or not using any round lenses at all.
Readers will find further objects and advantages of the invention from a consideration of the ensuing description and of the accompanying figures.